Hot section components of gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Air plasma spraying (APS) has the advantages of relatively low equipment costs and ease of application and masking, while TBC's employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures can be produced using other atomic and molecular vapor processes.
Observed failure mechanisms in turbine multi-layer systems are often anchored around interfacial challenges between the surface of the component and the TBC and/or different layers of the TBC. Such issues, including surface contamination, process inhomogeneity during start-up (e.g. inter-layer porosity, unmelts, etc.), and source cross-contamination can lead to interfaces with unreliable functionality, thereby endangering the multi-layer system's stability.
Thus, a need exists for multi-layered coating systems where individual layers can provide improvements to the coating system's damage tolerance, thermal properties, reactivity, etc.